feat: add receipt OCR pipeline (refs #69)

Implement receipt-specific OCR extraction for fuel receipts:

- Pattern matching modules for date, currency, and fuel data extraction
- Receipt-optimized image preprocessing for thermal receipts
- POST /extract/receipt endpoint with field extraction
- Confidence scoring per extracted field
- Cross-validation of fuel receipt data
- Unit tests for all pattern matchers

Extracted fields: merchantName, transactionDate, totalAmount,
fuelQuantity, pricePerUnit, fuelGrade

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ocr/app/extractors/__init__.py

@@ -1,10 +1,23 @@
 """Extractors package for domain-specific OCR extraction."""
 from app.extractors.base import BaseExtractor, ExtractionResult
 from app.extractors.vin_extractor import VinExtractor, vin_extractor
+from app.extractors.receipt_extractor import (
+    ReceiptExtractor,
+    receipt_extractor,
+    ReceiptExtractionResult,
+    ExtractedField,
+)
+from app.extractors.fuel_receipt import FuelReceiptExtractor, fuel_receipt_extractor
 
 __all__ = [
     "BaseExtractor",
     "ExtractionResult",
     "VinExtractor",
     "vin_extractor",
+    "ReceiptExtractor",
+    "receipt_extractor",
+    "ReceiptExtractionResult",
+    "ExtractedField",
+    "FuelReceiptExtractor",
+    "fuel_receipt_extractor",
 ]